In semiconductor manufacturing, etching is required to expose areas of the silicon substrate for diffusion or ion implantation of impurities so as to form integrated circuits in the silicon substrate. One method of etching is by process of plasma etching or reactive ion etching. In this process, a chemically reactive gas, such as CF.sub.4 is used. The surface of the silicon substrate to be etched is covered with a mask leaving selective areas of the surface exposed. The substrate with a surface to be etched is inserted into a chamber containing the reactive gas. To create the plasma, normally in RF voltage is applied across the gas to cause the gas to disassociate and form various positive and negative ions.
Highly reactive species, such as halogen and oxygen, especially in their atomic state, whether neutral or ionized, would be preferable in etching metals, silicon and oxides and nitrides of silicon. Atomic fluorine from plasma created from a gas of CF.sub.4 with a small percentage of oxygen is believed to react with silicon and the oxides and nitrides of silicon according to the following reactions: EQU CF.sub.4 .fwdarw.C+4F EQU 4f+si.fwdarw.SiF.sub.4 EQU 4f+siO.sub.2 .fwdarw.SiF.sub.4 +O.sub.2 EQU 12f+si.sub.3 N.sub.4 .fwdarw.3SiF.sub.4 +2N.sub.2
the etching or ashing of organic material such as photoresist also would be preferably done with atomic oxgen. A general reaction would be as follows: EQU C.sub.x H.sub.y +30.fwdarw.CO.sub.2 +H.sub.2 O
it is desirable to achieve a flow of etchant which is enriched with these reactive species toward the substrate to be etched. These highly reactive species are characterized by high electronegativity. However, if a positive potential is applied to attract these species to the substrate, electrons will also be attracted and the plasma will collapse.
Accordingly, the primary object of the present invention is to provide a novel process and apparatus which takes advantage of this electronegative characteristic for separating and extracting these highly reactive species as negative ions from the plasma while substantially retaining the free electrons in the plasma.
A more specific object of the present invention is to provide a process and apparatus for directing negative ions separated from a plasma to a surface, such as one to be etched.
A further object of the present invention is to be able to adjust the energy of the negative ions so as to control the directionality of the etching.
A still further object of the present invention is to provide a reactive ion etching process in which a large fraction of the negative ions directed to the surface to be etched are in the atomic state.
Another object of the present invention is to design the apparatus such that the structure, for retaining the free electrons within the plasma while passing the negative ions, is simple and easy to manufacture.